Piva,
If you are referring to your stop in the Panama Canal (Either in Gatun Lake or Cristobal) cruise passengers are usually prohibited from taking a tour on their own and must book shore trips through the cruise ship. The Gamboa Aerial Tram or the kayak trip are both good options, if they are offered. Some people have felt that the helicopter tour was well worth the money, while others have loved the train trip. Your interests will have a lot to do with what would be the best option. What cruise ship will you be sailing?
Nancy